Project Summary/Abstract A remarkable consensus is developing: the way physicians are organized and reimbursed in the United States must change. A leading example is the Medicare Shared Savings Program (MSSP) establishment of accountable care organizations (ACOs), groups of providers who assume collective responsibility for the spending and quality of a defined beneficiary population. The objective of this current project is to synthesize quasi-experimental design, novel data linkages, and primary survey methodology to examine the formation, effects, and mechanisms through which ACOs on spending and quality. We will accomplish this in three aims. First, we will identify key market factors that facilitate or inhibit ACO market penetration as represented by: (1) the presence or absence ACOs in a market, (2) the total number of ACOs, and (3) the share of beneficiaries covered by ACOs. We will examine these relationships for ACOs with both one- and two-sided risk models. Second, will assess the impact of ACOs on spending and quality via instrumental variable analysis. We will instrument for endogenous ACO participation by exploiting exogenous variation in the ?differential supply? of ACO vs. non-ACO physicians surrounding Medicare patients. Third, we will examine frontline physicians? attitudes and perspectives of ACOs through an in-depth survey of the Physician Organization of Michigan (POM) ACO, the largest ACO in the State of Michigan and the tenth largest in the country. Our survey will be one of the first and potentially largest examination of frontline physicians? attitudes and perspectives of ACOs, a perspective largely missing from policy narratives. We posit that ACOs will lead to modest in spending without corresponding decreases in quality but that individual physician uncertainty about ACO tasks and incentives will weaken collective incentives and harm the achievement of shared objectives. We expect our findings to inform Medicare in designing future generations of alternative payment models that are both more effective and more readily extended across the broader universe of Medicare physicians and patients.